To control the workplace sanitation and atmosphere of a CNC work environment, workpiece support tables for CNC cutters have been developed to contain a water bath or pool beneath the workpiece support surface. The support surface often comprises a multiplicity of flat bar or angle edges characterized herein as workpiece support rails.
In the example of plasma arc cutting, the “kerf” of a material cut from a whole is that portion of the material that is removed in fine particles, such as by a saw, or by melting, such as by a cutting torch. At 15,000° C. to 25,000° C., the plasma arc cutting process not only melts the kerf of a metal workpiece, but also partially vaporizes the metal. Consequently, metallic workpieces, especially material removed from within the kerf, become either solid material slag or noxious vapors. Fortunately, some of the metallic vapors produced by plasma arc cutting have an affinity to water solubility.
In another example, water jet cutters utilize high-pressure nozzles which direct water mixed with fine-grained abrasive particles (e.g., garnet, aloxite) to wear down, shape, and cut a workpiece. This results in wastewater in which the abrasive particles are mixed with fine particles from the kerf, which can often settle to the bottom of the water pool and clog up or otherwise impede circulation.
There are numerous reasons for changing or regulating the surface level of the water pool relative to the workpiece support rails. One very important reason is to physically remove the accumulated slag or abrasive that falls to the water pool bottom. The water surface level is lowered for physical access to the debris that has accumulated on the pool bottom. However, industrial CNC cutting tables are quite large, having dimensions in the order of 6 meters (m) long and 4.5 m wide. Consequently, the volume of water supported under the workpiece support rails is significant. Moreover, after a period of use, the water may be contaminated with considerable quantities of dissolved metals, abrasives, and other hazardous compounds. Accordingly, regulation of the water table surface level by merely discharging the water into municipal sewers and replacing with fresh water is neither an economic nor environmental option.
For these and other reasons, workpiece support tables for CNC cutting systems have included closed systems, having two or more water volumes that are isolated, but hydraulically linked, whereby water is transferred between a primary pool beneath the workpiece support rails and a secondary volume. Depending on the surface level required for the primary pool, water is transferred to or from the secondary pool. Obviously, such hydraulic transfers may be effected with bi-directional pumping. However, considering the frequency and volume of such fluid transfer cycles, it has been discovered that a closed air pressure pocket, formed over the secondary volume to push or withdraw fluid to or from the primary volume, is more effective and efficient. The primary volume can be placed at a greater elevation than the secondary volume to gravitationally drain the primary volume through the linking conduits into the secondary volume, when air pressure in the pocket is reduced. Conversely, when a rise in the primary volume surface level is required, pressure in the air pocket can be increased to push water up through the linking conduits into the primary volume, thereby raising the surface level.
Depending on the elevation difference between the primary and secondary volumes, the magnitude of air pressure in the pocket determines the height of the primary volume. However, if for some reason, such as slag accumulation, flow is obstructed, through the linking conduits between the primary and secondary volumes, then the level control system may increase the pocket pressure beyond reasonable limits in a futile attempt to raise the primary volume surface level. As a result, the equipment may be damaged through leakage, and in extreme cases, workplace accidents may result from radical overpressure leading to the pocket bursting and high-pressure fluid being vented.
An example of the prior art may be found in U.S. Pat. No. 3,743,260 to Alleman, which discloses one of the earliest surface variable water tables directed specifically to plasma cutting. The Alleman apparatus includes a tank assembly having a pair of false bottom plates beneath the workpiece support surface. These false bottom plates slope from the tank sides toward the tank center but are terminated short of the center to provide a center trough for slag collection. Beneath the false bottom plates is a true tank bottom. A wedge of volumetric space is formed between the underside of the false bottom plate, the tank wall, and the true tank bottom. This wedge of space is open to the water supporting volume above the false bottom plate. When the tank is filled with water, an isolated air chamber may be charged above the water surface within the wedge volume. Depending on the air pressure within the wedge volume, water is displaced from the wedge volume to raise the surface level of the upper tank volume. Control of the air volume within the wedge volume is by means of a manual valve (53), which requires careful supervision to avoid overpressure.
U.S. Pat. No. 5,013,884 to Hahn describes a plasma arc cutting system having a water filled chamber beneath the workpiece support surface. An air pressure expansible bladder is secured within the water volume. As the bellows is expanded by air pressure, a corresponding volume of water is displaced within the chamber. Within the fixed volume of the chamber, such displaced water volume may only be accommodated by the open surface adjacent the workpiece support surface.
The water surface level of a plasma cutting table as described by U.S. Pat. No. 6,387,320 to Poulin is controlled by means of a false bottom plate secured to the true tank bottom by means of a flexible skirt. A fixed water volume in the tank above the false bottom has a controlled surface level by means of air inflated bellows positioned between the underside of the false bottom and the upper face of the true bottom. Air pressure into the bellows expands the bellows length to raise the false bottom against the fixed water volume.
A need therefore exists for a CNC cutting table in which the pressure level between the fluid volumes is protected by a failsafe mechanism, thereby reducing the considerable safety hazard associated with precisely gauging the primary and secondary fluid volumes to avoid environmental hazards that can be created by overpressure (e.g., structural failure, such as leakage and tank bursts) and under-pressure (e.g., accumulation of toxins in the work environment). The invention disclosed herein meets these needs.